Typically, an optical relay transmission system performs control to absorb fluctuations or changes of loss in an optical fiber and to keep predetermined power of light in the optical fiber to maintain a satisfactory transmission performance, even when the loss fluctuates according to ambient temperatures, vibrations, or bends of an optical fiber between relay stations, or in each of a transmission station, a relay station, and a reception station.
When the optical relay transmission system is a wavelength-multiplexing transmission system, optical power in each of wavebands to be multiplexed needs to be kept at a predetermined value. For example, in Patent Document 1 below, a technology is disclosed that enables to control total power of a wavelength-multiplexed light in an optical fiber to have a value corresponding to the number of multiplexed wavelengths.
Patent Document 1: Japanese Patent Application Laid-open No. H6-21897